The invention relates to an improved apparatus for applying a full line voltage to start an electric motor, and, more particularly, to such a motor start apparatus including solid state switches for shunting the taps of an autotransformer to apply the line voltage to start the motor and additional solid state switches for protecting against a short circuit of the line voltage source when the taps of the autotransformer are positioned at low tap settings.
Polyphase synchronous or induction motors have been used to operate vibratory excitors and, for such applications, an autotransformer may be used to generate a reduced operational voltage for the motor.
the autotransformer comprises a plurality of coils that are connected in series and positioned across a relatively high line voltage source. Each coil of the autotransformer has an associated tap or wiper that is moved along the coil to vary the voltage that appears on the tap. The taps are connected to associated primary windings of the motor and the motor is operated at a speed corresponding to the voltages on the various taps of the autotransformer.
Typically, the motors of vibratory excitors are operated at a reduced voltage with respect to the line voltage and therefore, the taps of the coils of an associated autotransformer are adjusted to produce the lower operational voltage. However, although such motors are well adapted to operate at reduced voltages, the motors typically require a relatively high voltage to break from a rest position. Thus, if the autotransformer of a motor is adjusted to a reduced voltage and the motor is operated at the reduced voltage, when the motor is stopped, it is necessary to readjust the autotransformer of the motor to apply a relatively higher voltage to restart the motor. If many motors are used, for example, in a factory environment, such a voltage adjustment by the operator for many machines is time consuming and, therefore, expensive. Thus, it would be advantageous to provide a means to automatically apply an increased voltage to start such motors and to thereafter automatically reduce the operational voltage of the motors to a lower preset operational voltage value.
An automatic full voltage motor start system is disclosed in the patent to C. C. Nelson et al, U.S. Pat. No. 1,921,770. However, the circuit disclosed in the patent includes a multipole relay that is employed to shunt around the autotransformer of a motor to apply a full line voltage to start the motor. After the motor is started, the shunting relay is opened and another multi-pole switch is used to apply the line voltage to the coils of the autotransformer to generate a reduced operational voltage that is then gradually increased to the full line operating voltage.
Such a prior art voltage start circuit is complicated due to the fact that the line voltage input to the autotransformer must be switched in order to shunt the line voltage around the autotransformer. Thus, the prior art circuit necessarily employs a relatively complicated multi-pole switching apparatus to provide the increased voltage start function.
Accordingly, it is an object of the invention to provide a simple and effective switching apparatus for automatically applying a full line voltage to start a motor and automatically applying a preset reduced running voltage to operate the motor.
A further object of the invention is to provide such a switching apparatus wherein the autotransformer is not disconnected from the line input voltage when the switching apparatus switches to a full voltage start condition.
Another object of the invention is to provide such a full voltage start apparatus that includes means for avoiding transient current surges in the autotransformer when the full line voltage is applied to start the motor.
A further object of the invention is to provide a full voltage motor start apparatus that avoids a shorting of the line voltage supply when a full voltage is applied and taps of the autotransformer are set at low voltage positions.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.